In the field of semiconductor manufacturing, optical lithography has been the main stream approach to pattern semiconductor devices. In typical prior art lithography processes, UV light is projected onto a silicon wafer coated with a thin layer of photosensitive resist through a mask that defines a particular circuitry pattern. Exposure to UV light, followed by subsequent baking, induces a photochemical reaction which changes the solubility of the exposed regions of the photosensitive resist. Thereafter, an appropriate developer, usually an aqueous base solution, is used to selectively remove the resist either in the exposed regions (positive-tone resists) or, in the unexposed region (negative-tone resists). The pattern thus defined is then imprinted on the silicon wafer by etching away the regions that are not protected by the resist with a dry or wet etch process.
Many of the currently used resists are chemically amplified positive-tone resists which are made of two major components: The first component of such chemically amplified resists is an aqueous base soluble polymer where the polar functional groups are protected by acid labile protecting (or so-called blocking) groups. The presence of such protecting groups converts the aqueous base soluble polymer into an insoluble resin. Acid catalyzed deprotection of the protected sites converts the polymer back into an aqueous base soluble polymer.
The second major component of prior art chemically amplified positive-tone resists is a photoacid generator. Exposure of these resists to UV irradiation typically generates an acidic catalytic species as a result of the photochemistry of the acid precursor. The catalytic species are believed to induce a cascade of subsequent chemical transformations of the resist that alter the solubility of the exposed regions. Thus, the quantum efficiency of the photochemical event is amplified hundreds or even thousands of times through the catalytic chain reaction. The most commonly employed chemical amplification involves the acid catalyzed deprotection of various partially protected poly(p-hydroxystyrene), poly(acrylic acid), or copolymers thereof for positive-tone resists. The chemical amplification of resist materials are described in detail in U.S. Pat. No. 4,491,628 to Ito; U.S. Pat. No. 5,252,435 to Tani, et al.; U.S. Pat. No. 5,258,257 to Sinta, et al.; U.S. Pat. No. 5,352,564 to Takeda, et al.; U.S. Pat. No. 5,210,000 to Thackeray, et al.; and U.S. Pat. No. 5,585,220 to Breyta.
The acid labile protecting groups used with the aqueous base soluble polymer resins can be classified into two distinct groups: (I) High activation energy protecting groups such as t-butyl ester or t-butyl carbonyl groups; and (II) Low activation energy protecting groups such as acetal, ketal or silylether groups.
Resists containing polymer resins protected with high activation energy protecting groups generally require a post-exposure baking step to effect the deprotection and solubility switch due to the high bonding energy. In the case of resists containing low activation energy protecting groups, a post-exposure baking step is not necessarily required because of the facile deprotection chemistry of this group which generally occurs at about room temperature.
Accordingly, each of the foregoing acid labile protecting groups has their own advantages and disadvantages associated therewith. For example, the high activation energy protecting groups tend to give rise to high resolution, but suffer from environmental contamination during the post-exposure delay. On the other hand, the low activation energy protecting groups tend to exhibit robust environmental stability, however they normally suffer from line width slimming and shelf-life problems.
In view of the state of prior art resists, it would be beneficial if a new resist could be formulated which incorporates the advantages of both types of protecting groups in a single resist while eliminating the disadvantages associated with each protecting group. That is, there is a continued need to develop a new and improved resist which exhibits high resolution upon exposure to UV irradiation, good environmental stability and that could be used to pattern semiconductor circuitries.